Iron would be fine since there’s basically no atmosphere to oxidize it right?
And note that even what we call "cast iron" - a material that reasonably could be preferred to steel for some industrial purposes - is an iron-carbon alloy that in fact has more carbon than steel[1].
[0] https://www.texasironandmetal.com/strength-of-steel-compares...
it isnt always oxygen that does this, a difference of RedOx potential allowing redistribution of electrons is all you need.
mars has a perchlorate problem thus carbon compounds are converted to carbonate via Oxidation when encountering ubiquitous perchloate mineral deposits.
its toxic to carbon based biochemical forms, and destructive to carbon materials, such as carbon fibre; carbon nanotubes; carbon steel; even a lot of keypads.
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Pesky Perchlorates All Over Mars:
Cast iron is lighter than steel, not heavier, because of its higher carbon content.
However, objects made of cast iron are indeed heavier than similar objects made of steel, and this is what you must have in mind, because the objects made of cast iron are always made thicker, both because cast iron is weaker, which requires greater thickness for the same strength, and because it is harder to make thinner objects by casting than by forging.